Permanent magnets are traditionally made using rare earth materials (REMs) to achieve a high magnetization. With the dwindling resources, REMs are becoming more complex and expensive to source, thereby making permanent magnets more expensive. The dwindling resources of REMs are unable to match the demand for more permanent magnets, which have become homogenous in the design and function of modern electrical and electronic devices. This trend has given rise to extensive research in the use of ferromagnetic materials as an alternative to REMs. Ferromagnetic materials natively have a few advantages over REMs such as relatively higher magnetization and better thermal stability. Ferromagnetic materials, although cheaper and more abundant than REMs, have failed to act as an adequate replacement because of their high saturation magnetization, high Curie temperatures and low coercivity. The low coercivity is derived from their low magneto-crystalline anisotropy.
Making use of shape anisotropy of ferromagnets to develop coercivity has been explored previously. The best example of the same may be the Al—Ni—Co alloy (Alnico) permanent magnets that have been produced since the 1930s. In an Alnico magnet, the microstructure is primarily composed of two nano-scale phases formed through spinodal decomposition: isolated needles of ferromagnetic FeCo-rich phase and a non-magnetic matrix of NiAl-rich phase. However, performance of Alnico magnets is still restricted by their very modest coercivity (typical magnetic properties of commercial Alnico magnets have their coercivity Hci<1.5 kOe and energy product (BH)m<10 MGOe).
Extensive research in recent years in magnetic nanoparticles, especially in magnetic nanowires and nanorods, has renewed the interests in developing high coercivity in transition metal nanocrystals based on shape anisotropy. Electrochemical deposition and chemical synthesis are widely adopted to produce Co and Fe based ferromagnetic nanowires and nanorods with enhanced coercivity. Room-temperature coercivities up to 7.0 kOe have been reported for aligned single-crystalline Co nanorods.
What is needed is high coercivity exceeding 10 kOe at room temperature which will serve as ideal building blocks for future bonded, consolidated and thin film magnets with high energy density and high thermal stability.